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Unseptbium
Unseptbium, Usb, is the temporary name for element 172. NUCLEAR At least one set of theoretical values for half-lives and decay modes of Usb have been constructed for neutron count up to N = 333(1). It predicts isotopes ranging from Usb 440 to Usb 504 in three bands: Usb 440 to Usb 448, Usb 465 to Usb 486, and Usb 501 to Usb 504. Examination of pp 15 & 18 of Ref. 1 indicates that Usb 465 through Usb 486 are part of the expected pattern of alpha-decaying nuclei centered on the N = 308 shell closure. Isotopes between Usb 471 and Usb 481 are predicted to have sub-millisecond half-lives and to decay by alpha emission. The other isotopes in this band have sub-microsecond half-lives and most decay by alpha emission, which is consistent with neutron shell closure at Usb 480. Usb 440 to Usb 448 are very neutron-poor, and not located in the vicinity of any suggested shell closure; they appear to be artifacts of the sort common near the edges of models. Usb 501 to Usb 504 might indicate a shell closure near N = 330, but they are also located in a region whose patterns of half-lives and decay modes indicates that the model may have reached the limits of its capability. What Ref. 1 can’t do is describe heavy isotopes of Usb. It is possible to use a first-order, liquid-drop approach to guess at what lies there. At least two computations of the neutron dripline’s location up to Z = 175 exist(2),(3), and since they give similar results, the maximum possible size of a Usb nucleus can be set slightly above the values computed, allowing only a small margin for error. This gives Usb 615 as the heaviest possible Usb isotope. Similarly, a realistic lower bound can be set by requiring that the amount of energy needed to stabilize a nucleus be no more than twice what is needed to stabilize Usp 471. Within this range, the liquid-drop model can be used to indicate the amount of structural correction energy needed to allow a drop of nuclear matter to survive for the 10^-14 sec needed for electromagnetic interactions (such as binding an electron) to become important. Structural correction required for Usb 615 is nearly 1.5 MeV, which means all Usb drops will fission quickly without structural stabilization. In general, it is not possible to describe structural correction energy. What can be predicted are neutron and proton shell closures, for which correction energy is expected to be particularly large. Neutron shell closures have been predicted at N = 406(3),(4), 370(3), 318(5), and 308(1). The isotope Usb 578 requires 1.5 MeV of structural correction, which means isotopes in the Usb 568 to Usb 583 band are likely. (See “Formation” for additional significance of these nuclei.) Usb 542 requires more than 1.5 MeV of structural correction, which means isotopes in the band Usb 532 to Usb 547 are also likely. All isotopes in both bands should beta-decay with half-lives under a second. On the other hand, Usb 490 requires 5 MeV of correction energy, which means it is likely to stabilize some nuclei in its vicinity. Ref. 1 does not show a pattern of nuclides which indicate a shell closure at N = 318. Usb 480 requires 7 MeV of correction energy, which is realistic for a strong neutron shell closure, such as the one predicted at N = 308, so the liquid-drop picture isn’t unrealistic. ATOMIC Several predictions for the ground state electron structure of Usb agree that it will inert-gas character, with two 9s, two 9p1/2, and four 8p3/2 electrons available for bonding. Electrons in Usb may be describable in terms of time-independent orbitals, but there is a good chance that the conventional time-independent orbital concept does not apply to atoms with this high a value of Z. Calculation of electron properties require that nuclear charge be distributed over the nucleus' actual volume. In addition, there is some chance that differing nuclear shapes may produce different electron configurations in different isotopes. (Different isotopes would be different elements in the chemical sense.) Except in the laboratory, Usb is expected to exist only in environments too hot for ordinary chemistry to occur. FORMATION Ions of this element may form when material from roughly 1 km depth is ejected from a disintegrating neutron star during a merger. There is a possibility that beta decay from dripline nuclides stabilized by the N = 406 closure, enhanced by the Z = 164 proton shell closure, will allow some isotopes in the vicinity of Usb 567 to Usb 577 to form in quantity during such a merger. It improbable that nuclides between Usb 532 and Usb 547, or lighter, can form in this way. Fusion or multinucleon transfer reactions in the polar jets emanating from a neutron star or black hole might produce lighter isotopes, including those in the Usb 465 to Usb 486 band. Quantities produced by this method are very small. REFERENCES 1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011. 2. "Neutron and Proton Drip Lines Using the Modified Bethe-Weizsacker Mass Formula; D.N. Basu et al; Int.J.Mod.Phys.; arXiv:nucl-th/0306061; url: https://arxiv.org/abs/nucl-th/0306061 3. “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013. 4. "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v. 68, no. 7, pp 1133-1137; 2005. 5. “The Highest Limiting Z in the Extended Periodic Table”; Y.K. Gambhir, A. Bhagwat, and M. Gupta; Journal of Physics G: Nuclear and Particle Physics. 42 (12): 125105. DOI:10.1088/0954 3899/42/12/ 125105. (12-11-19)